Gene Therapy for Thoracic Malignancies
Treatment options for late stage lung cancer and malignant mesothelioma are limited and ineffective at improving prognosis. As such, gene therapy of these two malignancies are being explored as possible treatment modalities for increasing life expectancy and even reducing the volume and mass of these tumors to increase the chances for complete surgical resection. Gene therapy vectors in use/development for thoracic malignancies: *Adenoviruses (Figure 1) *Retroviruses *Lentiviruses *Adeno-Associated Viruses *Vaccinia/Fowl Pox Vectors *Nonviral Vectors *Antisense Therapy Suicide gene therapy Most chemotherapeutics have poor selectivity towards cancer cells and thus adversely affect normal cells that rapidly divide. To circumvent this issue and increase tumor cell killing, suicide gene therapy has been investigated for tolerance by patients and cell killing activity (1-4). Although variable results in terms of tumor regression were observed, treatment with the adenovirus mediated herpes simplex virus thymidine kinase which rendered local malignancies sensitive to gancyclovir was well tolerated (4,6). In a phase 1 clinical trial follow up, a patient was found to have long term survival as long as 10 years post treatment with Ad.HSV-TSK (Figure 2) (5). The long term success of this treatment modality was attributed to a likely immune bystander effect which resulted in the death of surrounding non-infected cancer cells along with the infected cells. Cytokine gene therapy When high levels of immunostimulatory cytokines (eg. IL-2, TNF, GCSF, etc.) are secreted by cells, immune cells surounding these secretory cells are activated to clear the source of these cytokines and resolve the perceived infection/damage without the presence of an active infection. Harnessing this rationale, researches investigated the possiblility of activating immune cells at the sites of tumors by delivering the immunostimulatory cytokines directly to the tumors via viral vectors (6,7). In one such clinical trial 6 patients with refractory mesothelioma were treated with an intratumoral injection of a replication restricted vacinnia vector for IL-2 for 12 weeks (7). The production of antibodies against the vaccinia virus was monitored in the patients as well as people who had contact with them in order to ensure that the virus was not being transmitted and was safe for use as a vector. The antibody titers were found to increase in the patients only with no adversde side effects. Mukherjee et al., observed VV-IL-2 expression for 3 weeks after initial delivery and infiltration of Tcells into 50% of the tumors they biopsied. They thus concluded that IL-2 delivery by the vaccinia virus would be safe and could yield positive results even though no tumor regression was observed in their study. In a different study, patients were given monkey fibroblast cells (Vero cells) expressing human IL-2 as a course of four injections. A reduction in tumor size and volume was observed in these patients and the treatment appeared to be well tolerated (cross referenced from 6). Gene-Modified Dendritic Cell-Based Vaccination In this approach, immature dendritic cells (DCs) are either isolated from patient blood monocytes and maturred ex vivo by loading them with purified protein antigens, tumor cell extracts or mRNA befor administering them back to the patient subcutaneously. DCs have also been modified by transfecting them with cytokine or chemokine vectors before injecting them directly back into patient tumors where they can assimilate antigens and present them to T cells after migration to lymph nodes in order to elicit an immune response (6). Purified proteins that have been used for maturing DCs before injecting them back into patiens include p53 which is usually expressed at low levels in normal cells but its mutant form is overexpressed in tumor cells and thus is useful as a tumor vaccine to sensitize the immune system (8). In a phase I study using this mode of treatment, small cell lung cancer (SCLC) patients receiving DCs transduced with Ad.p53 showed varying results (8). 62% of the patients had an immune response to the treatment and responded positive to second line chemotherapeutics at a higher rate than expected (8). In a preclinical study on the effectiveness of DCs transduced with the chemokinee CCL21 that promotes Tcell maturation, showed that CCL21 transduced DC a robust effect on lung cancer (9) Gene-Modified Tumor Vaccines By employing genetic modification approaches, researchers are expressing genes in irradiation killed tumor cells to make them more immunogeneic (6). Some of the following approaches are being used to make tumor cells more immunogenic before they are irradiated and used as vaccines: *Transforming growth factor-β2 (TGF-β2) antisense vector–modified cells (10) *Granulocyte-monocyte colony-stimulating factor (GM-CSF) transfected tumor cells (11) *B7.1/HLA vaccination (12) *α(1,3)Galactosyltransferase (13) Conclusion A lot of advances have been made in gene therapy for the treatment of thoracic malignancies but the biggest limitation is longterm efficacy. Administration of vectors to all the cells of a tumor has posed a big hurdle that needs to be crossed. Although viral vectors that promote secretion of the gene products they encode would be a useful resource in gene therapy, vectors that promote long term expression of the gene are required. Therapies that target the tumor by increasing immunogenecity are currently being favored; however, large tumor burdens and tumor driven compensatory mechanism that inhibit immunosurveillance and immunogenecity are countering these efforts (6,14). References 1. Smythe WR, Hwang HC et al., (1994) Use of recombinant adenovirus to transfer the herpes simplex virus thymidine kinase (HSVtk) gene to thoracic neoplasms: an effective in vitro drug sensitization system.Cancer Res 54(8):2055-9 pdf 2. Hwang HC, Smythe WR, Elshami AA et al., (1995) Differential sensitivity of thoracic malignant tumors to adenovirus-mediated drug sensitization gene therapy. J Thorac Cardiovasc Surg 109(4):626-30 3. Hwang HC, Smythe WR, Elshami AA et al., (1995) Gene therapy using adenovirus carrying the herpes simplex-thymidine kinase gene to treat in vivo models of human malignant mesothelioma and lung cancer. Am J Respir Cell Mol Biol. 13(1):7-16. 4. Sterman DH, Treat J, Litzky LA et al., (1998) Adenovirus-mediated herpes simplex virus thymidine kinase/ganciclovir gene therapy in patients with localized malignancy: results of a phase I clinical trial in malignant mesothelioma. Hum Gene Ther. 9(7):1083-92. pdf 5.Sterman DH, Recio A, Vachani A et al., (2005) Long-term follow-up of patients with malignant pleural mesothelioma receiving high-dose adenovirus herpes simplex thymidine kinase/ganciclovir suicide gene therapy. Clin Cancer Res. 11(20):7444-53. 6. Vachani A, Moon E et al., (2010) Gene therapy for Mesothelioma and Lung Cancer. Am J Respir Cell Mol Biol. 42(4):385-93 7. Mukherjee S, Haenel T, Himbeck R, Scott B, Ramshaw I, Lake RA et al., (2000) Replication-restricted vaccinia as a cytokine gene therapy vector in cancer: persistent transgene expression despite antibody generation.Cancer gene therapy 7(5):663-70 8. Antonia SJ, Mirza N, Fricke I, Chiapori A et al., (2006) Combination of p53 Cancer Vaccine with Chemotherapy in Patients with Extensive Stage Small Cell Lung Cancer. Clin Cancer Res. 12:878-887 (pdf) 9. Baratelli F, Takedatsu H, Hazra S, Peeble K et al., (2008) Pre-clinical characterization of GMP grade CCL21-gene modified dendritic cells for application in a phase I trial in Non-Small Cell Lung Cancer. Journal of Translational Medicine 6:38 doi:10.1186/1479-5876-6-38 10. Nemunaitis J, Dillman RO, Shwarzenberger PO Senzer N et al., (2006) Phase II Study of Belagenpumatucel-L, a Transforming Growth Factor Beta-2 Antisense Gene-Modified Allogeneic Tumor Cell Vaccine in Non–Small-Cell Lung Cancer. J Clin Oncol 24(29):4721-30 11. Nemunaitis J, Sterman D, Jablons D, Smith JW 2nd et al., (2004) Granulocyte-macrophage colony-stimulating factor gene-modified autologous tumor vaccines in non-small-cell lung cancer. J Natl Cancer Inst. 96(4): 326-31 12. Raez LE, Cassileth PA, Schlesselman JJ, Sridhar K et al., (2004) Allogeneic vaccination with a B7.1 HLA-A -modified adenocarcinoma cell line in patients with advanced non-small-cell lung cancer.J Clin Oncol 22(14):2800-7 13. Morris JC, Janik JE, Vahanian N, Mertes S et al., (2006) A phase I study of antitumor vaccination using tumor cells genetically modified to express alpha(1,3)galactosyltransferase (GT) in patients with refractory or recurrent non-small cell lung cancer (NSCLC). J Clin Oncol. 24(18) supplemental 12503 14. Tagawa M, Tada Y, Shimada H and Hiroshima K (2013) Gene therapy for malignant mesothelioma: Current prospects and challenges. 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